The materials of the present invention are most importantly substrate candidates for electronic devices. Several processes in the manufacture of electronic devices such as liquid crystal displays (LCDs), solar cells, electronics, microelectronics etc. include steps that are performed at extremely high temperatures. For example, active matrix LCDs employ an active device such as a diode or thin film transistor at each pixel thereby enabling high contrast and high response speed. Although many display devices currently utilize amorphous silicon (a-Si), the processing of which may be accomplished at temperatures under 450° C., polycrystalline-silicon (poly-Si) processing is preferred. Poly-Si has a much higher drive current and electron mobility thereby increasing the response time of the pixels. Further, it is possible, using poly-Si processing, to build the display drive circuitry directly on the glass substrate. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques. The most efficient poly-Si processing methods operate at temperatures of at least 730° C., such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. This fabrication process typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in the substrate being heated to temperatures in the range of 650° C. or higher. Common commercial LCD glasses (e.g. Corning 1737 and Corning Eagle) exhibit strain points of approximately 670° C. Fused silica has a sufficiently high strain point of 990-1000° C., but its coefficient of thermal expansion (C.T.E.) of 5×10−7/° C.) is significantly lower than that of silicon which has a C.T.E. of 37×10−7/° C.) which can lead to high stress and failure. Further, the cost associated with formed fused silica substrates suitable for electronic devices is a deterrent. The strain point of most LCD glasses can be increased by lowering the modifier content of the glass and increasing the silica content, but this also raises the temperature required to melt and fine the glass to a high quality melt. This temperature is often referred to as the 200 Poise temperature or T200P. Thus generally, the higher the strain point, the higher the T200P, which accelerates corrosion of the refractories, increases energy consumption, and the overall cost, so there is often a tradeoff between strain point and meltability.
For other electronic devices, common processing steps also require high temperature substrates to withstand processing. Most high level electronic fabrication requires annealing of the gate oxide and dopant activation. These processes occur at temperatures in excess of 650° C.
Even in the case of single crystal silicon (x-Si) fabrication techniques that employ a thin layer of single crystal silicon bonded to a substrate, high temperature substrates are required. Single crystal silicon allows for even greater electron mobility than that achieved with poly-Si. The bonding step often requires high temperatures as well as the gate oxide and dopant activation steps previously described.
Liquidus viscosity also plays a major role in glass selection for substrate candidates. Lower liquidus temperatures translate into higher liquidus viscosities. These high viscosities allow for a large selection of commercially relevant forming techniques such as downdraw techniques. One particular example of a downdraw technique is known as the overflow downdraw or fusion sheet manufacturing process. The overflow downdraw process is described in U.S. Pat. No. 3,338,696 and U.S. Pat. No. 3,682,609. Glasses with low liquidus temperatures, allowing for high viscosities in the forming apparatus, are therefore good candidates for downdraw manufacturing processes. Lower liquidus temperature glasses also have the advantage of causing less corrosion on the refractory materials used in the forming processes. This translates into longer life for the forming apparatus, while a glass with a low melting and fining temperature (T200P) increases tank life.
A need exists, then, for a glass that (1) has a high strain point (>650° C.), (2) does not require costly heat treatments after fabrication, (3) has a CTE close to that of silicon, and (4) can be melted in a conventional melting unit (T200P<1650° C.) and formed according to a commercially proven method. In addition, the glass will preferably be transparent to visible radiation and be chemically durable. These several qualities are needed in glasses for production of such varied products as flat panel displays, photovoltaic cells, photomasks, optomagnetic disks and tubing and fiber applications that require stability at high temperatures.
A primary purpose of the present invention is to provide an alkali-free glass that has properties suited to production of a poly-Si or x-Si coating on its surface.
Another purpose is to produce a glass having a sufficiently high strain point to permit processing at temperatures in excess of 650° C.
A further purpose is to provide a glass that can be melted and formed by conventional procedures, and that can provide a substrate for application of a high quality, poly-Si or x-Si film.
A still further purpose is to provide an electronic device, in particular, a flat panel display, and having a high-quality, poly-Si or x-Si, thin film on its surface.
Another purpose is to provide a novel glass family consisting essentially of RE2O3—Al2O3—SiO2, with RE comprising a rare earth, and optionally containing selected oxides such as alkali, alkaline earth, and transition metal oxides.